1. Field of the Invention
The present invention relates to a novel organic electroluminescence (hereinafter referred to as EL) device. More particularly, the present invention relates to an organic EL device having a specific construction in which the optical thickness from an anode to a cathode is controlled and the color purity of the blue light emission is particularly increased.
2. Description of the Related Arts
EL devices show high self-distinguishability because of the self-emission, and are excellent in impact resistance and easy handling because they are completely solid devices. Therefore, EL devices have been attracting attention for application as light emitting devices in various types of display apparatus.
The EL device includes an inorganic EL device in which an inorganic compound is used as the light emitting material, and an organic EL device in which an organic compound is used as the light emitting material. The organic EL device has been extensively studied for practical application because the applied voltage can be decreased to a large extent.
As for the construction of the organic EL device, the basic construction comprises an anode/a light emitting layer/a cathode. (This description shows that an anode, a light emitting layer, and a cathode are laminated in this order. Other constructions are described similarly.) Constructions having a hole injecting and transporting layer or an electron injecting and transporting layer suitably added to the basic construction are known. Examples of such construction are a construction of an anode/a hole injecting and transporting layer/a light emitting layer/a cathode and a construction of an anode/a hole injecting and transporting layer/a light emitting layer/an electron injecting and transporting layer/a cathode. The hole injecting and transporting layer has the function of transporting holes injected from the anode to the light emitting layer. The electron injecting and transporting layer has the function of transporting electrons injected from the cathode to the light emitting layer. It has been known that, when the hole injecting and transporting layer is inserted between the light emitting layer and the anode, more holes are injected into the light emitting layer in a lower electric field, and electrons injected into the light emitting layer from the cathode or the electron injecting and transporting layer are accumulated at the interface between the hole injecting and transporting layer and the light emitting layer because the hole injecting and transporting layer does not transport electron. As the result, efficiency of the light emission is increased.
In the organic EL devices described above, various attempts have been made to increase the efficiency and the luminance by controlling thickness of various layers in the multi-layer organic part placed between the anode and the cathode, such as the hole injecting and transporting layer, the light emitting layer, and the electron injecting layer.
For example, it was disclosed that, in a construction of an anode/a light emitting layer having the hole transporting property/an electron transporting layer/a cathode, the efficiency of the light emission was increased by controlling thickness of the electron transporting layer in the range of 30 to 60 nm (Japanese Patent Application Laid-Open No. Heisei 4(1992)-137485. This technology showed that the distance between the light emitting layer and the cathode is an important factor. In another technology, thickness of the electron transporting layer is controlled in such a manner that the intensity of the light is substantially enhanced with the light emitted directly from the light emitting layer and the light reflected at the cathode interfere with each other (Japanese Patent Application Laid-Open No. Heisei 4(1992)-328295).
However, these technologies described above did not disclose that color purity of a device can be improved by suitably selecting thickness of the layers including organic layers placed between two reflecting interfaces. Furthermore, it is necessary in these technologies that the thickness of the electron transporting layer is controlled, and due to this requirement, the electron injecting layer influences the light emission process and cause unfavorable results, such as deterioration of the color purity and decrease in the efficiency. Therefore, improvement to eliminate these problems has been required.
An EL device having a construction in which a metal oxide is inserted between an anode and a transparent substrate was disclosed (Japanese Patent Application Laid-Open No. Heisei 4(1992)-334895). However, the layer of a metal oxide is formed with the object of shielding the organic layers of the EL from ultraviolet light which causes degradation of the organic layers. Therefore, this technology does not suggest the present invention.
An EL device having a construction of an anode/a hole injecting layer/a light emitting layer/an electron injecting layer/a cathode and using a mixed layer containing a specific metal complex and an organic compound different from the complex as the electron injecting layer, was also proposed (Japanese Patent Application Heisei 5(1993)-96407. However, optical thickness of the organic multi-layer part in this technology is not set at a value which enhances intensity of a selected wave length of the emitted EL light. Any suggestion for this particular point is not included, either.
It has been known that, in an EL device having a construction of a substrate/a multi-layer film of dielectrics/a transparent electrode I an organic multi-layer part/a cathode, color purity can be increased by controlling the total optical thickness of the transparent electrode and the organic multi-layer part. However, this technology requires that the multi-layer film of dielectrics be used and it is inevitable that cost is increased.
On the other hand, there was disclosed a technology adopting a construction of a transparent electrode/a dielectrics layer/a fluorescent substance layer/a dielectrics layer/a back electrode and adjusting thickness (d) of a fluorescent substance layer or a laminate containing a fluorescent substance layer and a dielectrics layer and refractive index (.eta.) to satisfy the equation: d=k.multidot..eta..multidot..lambda./2 (.lambda. being wave length of the emitted light) (Japanese Patent Application Laid-Open No. Heisei 2(1990)-46695). In this technology, an inorganic fluorescent substance, such as ZnS, is used as the fluorescent substance layer, and an insulation film containing an oxide and the like is used as the dielectrics layer. The thickness of the fluorescent substance layer or the laminate of the fluorescent substance layer and the dielectrics layer is adjusted in accordance with the refractive index in such a manner that the light emitted from the fluorescent substance layer is reflected multiple times between the interface between the transparent electrode and the dielectrics layer and the interface between the dielectrics layer and the back electrode, and the reflected lights interfered with each other. However, the interface between the transparent electrode and the dielectrics layer is treated as a reflecting interface in this construction because the refractive indices of the dielectrics layer and the fluorescent substance layer are 2.0 or more and the refractive index of the transparent electrode is about 1.8 or less. In contrast, the refractive index of the organic multi-layer part is 1.6 to 1.8 in the construction of the transparent electrode/the organic multi-layer part/a cathode disclosed in the present invention. The interface of the transparent electrode and the organic multi-layer part cannot be treated as a reflecting interface. Japanese Patent Application Laid-Open No. Heisei 2(1990)-46695 described above does not suggest that the interface between the transparent electrode and the substrate, the interface between the transparent electrode and the underlayer of a high refractivity, or the interface between the transparent electrode and the underlayer of a low refractivity, can be treated as the reflecting interface, and the total optical thickness of the transparent electrode and the organic multi-layer part is controlled.